Strained semiconductor materials advantageously provide improved electrical carrier mobility properties as compared to relaxed semiconductor materials, thus increasing the speed at which semiconductor circuits can operate. A semiconductor layer is said to be “strained” when it is constrained to have a lattice structure in at least two dimensions that is the same as that of the underlying single crystal substrate, but different from its inherent lattice constant. Lattice strain occurs because the atoms in the deposited film depart from the positions normally occupied when the material is deposited over an underlying structure having a matching lattice structure. The degree of strain in a deposited layer is related to several factors, including the thickness of the deposited layer and the degree of lattice mismatch between the deposited material and the underlying structure.
Strained semiconductor layers can be formed by epitaxially depositing silicon over a silicon germanium layer. Silicon germanium (Si1-xGex, where 0≦x≦1) films are used in a wide variety of semiconductor applications, such as in microelectronics fabrication. Because silicon germanium has a larger lattice constant than silicon, when epitaxial silicon germanium deposition occurs over silicon (such as during deposition on a silicon wafer), the epitaxially deposited silicon germanium is “strained” to the smaller underlying silicon lattice. If a strained silicon layer is to be deposited over the silicon germanium layer, the silicon germanium layer should first be “relaxed” to its native lattice dimensions so that the silicon layer deposited thereover will be strained. In particular, because a strained silicon germanium layer has the dimensions of the underlying silicon lattice, a silicon layer deposited over a strained silicon germanium layer will not be strained. In contrast, a silicon layer deposited over a “relaxed” silicon germanium layer will be strained to conform to the larger underlying silicon germanium lattice. Thus, a strained silicon layer can be produced by epitaxially depositing silicon over a relaxed silicon germanium layer.
As the thickness of a strained silicon germanium layer increases beyond a “critical thickness”, defects in the crystal structure of the strained silicon germanium layer appear, thereby inducing relaxation. After relaxation occurs, the degree of strain present in the silicon germanium layer is related to the amount of misfit dislocation generated in the layer during relaxation, which is a function of the elastic energy of the layer and the activation energy for dislocation nucleation and gliding. The critical thickness depends on a variety of factors, including growth rates, growth temperature, germanium concentration, and the number of defects within the layer underlying the silicon germanium layer. Unfortunately, relaxation often accompanies vertically propagating threading dislocations, which can adversely affect device operation.